The present invention relates to a microscopic capacitance measurement system capable of measuring a microscopic capacitance value with a high precision of the order of several femtofarads by removing any influence by a floating capacitance.
FIG. 1 schematically shows the structure of a conventional capacitance measurement system for measuring a capacitance value of a capacitor formed on a silicon substrate. The measurement system is provided with a prober 1 and a capacitance meter 2. The prober 1 has a grounded shield box 11. In the shield box 11, a grounded stage 13 is provided for mounting a measured sample which forms a capacitor 12 to be measured. A manipulator 14 is disposed on a base surrounding the stage 13, and conductive portions of the manipulator 14 are grounded. Out of coaxial cables 3, 4 having the outer conductors grounded by electrically connecting to the shield box 11, one end of the inner conductor 31 of one the coaxial cables 3 is connected to a measurement electrode which faces the measured sample to form the capacitor 12 to be measured, and the other end of the inner conductor is connected to a detection terminal (input terminal) of the capacitance meter 2. The inner conductor acts as a signal line interconnecting the measurement electrode and the capacitance meter 2. One end of the inner conductor 41 of the other coaxial cable 4 is connected to the measured sample, and the other end thereof is grounded. In this embodiment, the manipulator 14 includes an actuator for adjusting the positions at which one ends of the inner conductors of the coaxial cables 3, 4 contact with the measurement electrode and the measured sample. The prober 1 is also provided with a microscope.
FIG. 2 shows in an enlarged manner how one ends of the respective inner conductors of the coaxial cables 3, 4 are in contact with the measurement electrode and the measured sample. On one surface of a silicon substrate 121, the sample to be measured is provided as a first layer electrode 123 via a field oxide film 122, and a second layer electrode 125 is disposed on the first layer electrode 123 via a capacitive insulating film 124, thereby forming the measured capacitor 12 between the first layer electrode 123 and the second layer electrode 125. A back surface of the silicon substrate 121, i.e., a surface which comes in contact with the stage 13 is grounded. During the measurement, one end of the inner conductor 31 of the coaxial cable 3 contacts with the second layer electrode 125, and one end of the inner conductor 41 of the coaxial cable 4 contacts with the first layer electrode 123.
Since a conventional capacitance measurement system is constituted such as described above, floating capacitances are formed between the shield box 11 and the conductive portions in the prober 1, between the shield box 11 and the conductive portions of the manipulator 14, between the shield box 11 and the stage 13, between the stage 13 and the back surface of the silicon substrate 121, and the like. Therefore, a total value of these floating capacitances is added to a capacitance value of the measured capacitor. In order to avoid an erroneous measurement caused by the presence of such floating capacitance in the prior art, the output of the capacitance meter 2 when one end of the inner conductor 31 or 41 does not contact with the second layer electrode 125 is beforehand obtained as an error value, and this error value is subtracted from actually measured capacitance values thereby correcting the measurement values.
However, such a floating capacitance is changed greatly (e.g., on the order of several hundreds of femtofarads) by various factors such as:
how extent the coaxial cables 3, 4 are folded;
how extent the dielectric constant of the insulating layer within each of the coaxial cables 3, 4 changes with temperature;
a mutual positional relationship among conductive portions of the coaxial cables, the manipulator 14 and the like in the shield box 11;
how extent the dielectric constant of the air within the shield box 11 fluctuates with temperature; and
a movement of measuring persons.
As a result, a capacitance of about several tens of picofarads is an upper limit to be measured by the conventional capacity measurement system, and it is impossible to measure a microscopic capacitance of several tens of famtofarads or less.
The present invention has been proposed to solve the problems above-described, and an object of the invention is to provide a probing and measurement system capable of providing a highly precise measurement value by almost removing any influence of a parasitic capacitance.
To achieve the above-mentioned object, the present invention provides a probing and measurement system comprising:
a prober having a box in which a sample to be measured is disposed, and which comprises a signal line having one end which is a detection terminal for contacting with said sample to be measured, and a shield line surrounding said signal line;
device for placing at least one of a conductive portion of said box or a predetermined conductive portion of said sample to be measured and said shield line at the same electric potential; and
a capacitance measurement circuit comprising an operational amplifier which has an inverting input terminal connected to the other end of said signal line and a non-inverting input terminal connected to said shield line, wherein an imaginary short state exists between said inverting input terminal and said non-inverting input terminal and wherein a signal having a value corresponding to an electrostatic capacitance of said sample to be measured is outputted when an AC signal is applied to said non-inverting input terminal,
whereby any influence by a parasitic capacitance and fluctuation thereof within said shield box is almost removed.
It is noted here that the box is preferably a shielded box. Further, the box is not limited to be box-shaped and may be any container as long as the box can accommodate the prober. The predetermined conductive portion of the sample to be measured is preferably all the conductive portions other than an electrode to be measured at least at a certain moment, excluding an earth (ground) electrode. Actually, it is difficult to interconnect all of the conductive portions, and, therefore, the predetermined portion of the measured sample may be a probe card or another conductive portion connected to a probe, excluding the electrode to be measured, or may be all or a part of the conductive portions connectable via any possible means, other than the electrode to be measured.
An electrostatic capacitance is formed between at least two electrodes of the sample to be measured, and the detection terminal contacts with either one of the electrodes of the sample to be measured.
The shield line preferably surrounds the entire length of the signal line, except the detection terminal.
It is preferred that the probing and measurement system further comprises:
a grounded signal line having one end being in contact with the other electrode of the sample to be measured, and the other end being grounded; and
a shield line surrounding the grounded signal line, and electrically connected to the shield box.
The prober preferably comprises a manipulator for causing one end of the signal line and one end of the grounded signal line to be in contact with appropriate positions of the electrode and the sample to be measured. A predetermined conductive portion of the prober is placed at the same electric potential as the shield box.
The predetermined conductive portion of the prober means a part or all of the portions other than the detection terminal and the signal line therefor. The sample to be measured is, for example, a semiconductor wafer.
It is appreciated that the present invention is advantageous in that it is possible to obtain an output which depends merely upon a capacitance value of a measured electrostatic capacitor, without any influences by any parasitic capacitance considered to be formed between the signal line and the shield line surrounding thereof, such as a parasitic capacitance within the shield box and a fluctuation thereof, whereby a measured capacitance value can be detected with a high precision even if the capacitance value is microscopic.
The measurement result by the present invention has confirmed that measurement can be performed with a precision of several femtofarads. Further, in the case where a sample to be measured is a semiconductor wafer, various microscopic capacitances of the semiconductor wafer can be measured highly precisely, thereby enabling a high-performance inexpensive semiconductor device to be provided.
The above and other objects and advantages of the present invention will become apparent when reading the following description of the invention with reference to the accompanied drawings.